Everything I've read suggests all the digital assets from the DE still exist. They just need to be re-rendered in high definition.
Correct. The company that did the CG for the remaster, Foundation Imaging, is defunct, however the digital artists that worked on it have the files in their personal collections. One of them is Daren Dochterman, he has repeatedly said he's ready willing and able to redo the DE CGI if Paramount ever came calling.
Daren has also said that the newly color timed copy of TMP exists somewhere in Paramount's vaults, since they struck a new master of it as a starting place for the DE release.